Look out for Delta Cephei

Aileen O’Donoghue

Northwest of the Demon Star in Perseus of the Dec. 6 column is another variable star to watch, Delta Cephei (SEH-fee). This star is the prototype of the Cepheid (seh-FEE-id) Variable stars that Edwin Hubble used in the 1920s to expand the universe well beyond the bounds of our home galaxy, the Milky Way.

As shown in Figure 1, Cepheus is in the northwest sky in the early evenings, above and to the right of the bright stars Deneb (DEN-ebb) and Vega (VEE-guh) and below Cassiopeia and Perseus. It represents the king of Aetheopia, husband of Cassiopeia. He’s also the father of Andromeda whom he had chained to a rock for the sea monster Cetus to devour in exchange for not having his kingdom destroyed by the Poseidon for Cassiopeia’s boast that Andromeda was more beautiful than any of the Nereids. Perseus, however, rescued and married Andromeda, so Cepheus is also his father-in-law.

Though none of the stars in Cepheus really catch our eyes between its bright neighbors of Cassiopeia and Cygnus, some are quite remarkable and worthy of knowing. Its mnemonic stick figure is a house drawn by a child with a steep, peaked roof and the bottom left corner represented by a small triangle of stars instead of a single star. Er Rai (urr-RYE) at the peak of the roof will become increasingly important to Earth dwellers in the next millennium. For us, the north celestial pole is marked by Polaris, but Earth’s axis, like that of any spinning top, precesses, making the axis scribe a circle on the sky over 26,000 years. In 3942 BC, Thuban (THEW-ben) marked the pole. In about 3000 AD, Er Rai will be closer to the pole than Polaris and in about 4000 AD, it will be at its closest position to the pole and be that generation’s Polaris.

Two of the largest stars known are found in Cepheus. Mu Cephei, also known as Hershel’s Garnet Star and Erakis (EAR-rack-iss), is remarkably red in binoculars. A swollen red giant star in its death throes, it has been measured to be 1,650 times larger than the Sun … if it were to replace the Sun, the outer layers of the star would be midway between Jupiter and Saturn! But, just lower in the house than the brightest star in the middle, Kerhah (KUR-hah), is 1,900 times the radius of the Sun! That’s 8.5 times Earth’s distance from the sun so its outer layers would be almost as close to Saturn as the sun’s are to us.

The most important star in Cepheus for modern astronomy, however is the faint Delta at the peak of the triangle marking the lower left corner. In 1784, a deaf, 17 year-old astronomer, John Goodricke, discovered that Delta Cephei varied over a period of 5.366 days. As shown in Figure 2, its variation is not smooth, but a jagged “sharks fin” with a dramatic rise and slow decline in brightness. Other stars were then observed to have this same pattern and classified as Cepheid variables.

The variation is due to competing forces within the star. The life of all stars is a competition between the compressional force of gravity and explosive force of nuclear fusion. These forces are in dynamic equilibrium during the main part of a star’s life. As the stars age, however, the nuclear fires weaken and they go through times when the balance shifts from one force to the other, making them vary in brightness before gravity ultimately wins.

Cepheid Variables are rich in helium (the first product of nuclear fusion). As an atom, with both electrons bound to the nucleus, it is mostly transparent to light. Remove those electrons with heat and collisions (ionization), however, and helium becomes opaque, absorbing light and heating up instead of letting it pass. In Delta Cephei, its slow decrease in brightness is due to the helium becoming opaque as it is ionized. At its most opaque, the helium becomes so hot that it rapidly expands. The expansion cools the helium so that the electrons re-join the nuclei making it transparent. Initially, the expansion is very rapid and huge amounts of light are released as transparency proceeds quickly, causing the peak in brightness. Ultimately, though, the expansion cools the star so much that gravity is again able to compress it, heating and ionizing the helium to again dim the star in our sky.

Delta Cephei, at its brightest, is about as bright as Zeta at the right of the small triangle. At its dimmest, it is about as bright as Epsilon the left base. To see this, compare Delta Cephei at its minimum to Epsilon at 6:30 p.m. tonight (12/20). Tomorrow night, wait until 12:30 a.m. (on Thursday) to compare it at its maximum to Zeta. If you miss these, try for 1/5/17 8:52 p.m. and 1/7/17 2:52 a.m. To just see the maxima, look on New Year’s Day at 6:04 p.m. and 1/17/17 a 8:28 p.m. Beyond this, Google “Delta Cephei Maxima 2017” to find a page in Swedish with the times in Swedish Standard time EST+6 hours).

The most important property of Cepheid Variables was discovered by a woman, Henrietta Swan Leavitt of Harvard College Observatory. In 1912, she had cataloged over 1777 variable stars in the Magellanic Clouds of the southern hemisphere. When she arranged these stars by their period of variability, she started to realize this also ordered them by brightness: the brighter stars varied more slowly than the dimmer ones. This meant that astronomers can calculate the intrinsic brightness of a Cepheid Variable simply by measuring the period of variation!

In the 1920s, the “Great Debate” in astronomy was whether the spiral nebulae such as the Andromeda Galaxy were other galaxies, “Island Universes,” or objects within the Milky Way. Until then, it had been thought that the Milky Way, WAS the universe. Edwin Hubble, in 1925, used Cepheid Variable stars to calculate the distance to the Andromeda galaxy as 1.5 million light years. Though it’s now known to be 2.5 million light years away, Hubble’s measurements settled the debate, dramatically expanding the universe.

As you observe this seemingly unremarkable star, you can see one of the most important tools humanity has used to measure the universe from our tiny base.

The astronomers of the Adirondack Public observatory will be happy to focus their telescopes on the stars of Cepheus and other wonders from our Roll Off Roof Observatory (RORO) above Little Wolf Pond in Tupper Lake. Currently, we’re open to the public on the first and third Fridays of each month, weather permitting of course. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO (Roll Off Roof Observatory) at 518-359-6317 to talk with one of our astronomers. Observing starts about one half hour past sunset.